Ammianus Marcellinus was a Roman historian who wrote during Late Antiquity.
He was "a soldier and also a Greek" ut miles quondam et graecus (A.M. 31.16.9) he tells us, and his enrollment among the elite protectores domestici (household guards) shows that he was of noble birth. He entered the army at an early age, when Constantius II was emperor of the East, and was sent to serve under Ursicinus, governor of Nisibis in Mesopotamia, and magister militiae.
More on [ Ammianus Marcellinus ]

Ancient History Sourcebook: The Luxury of the Rich in Rome - An excerpt from the Roman History XIV.16, from the Davis translation, in which Ammianus criticizes Rome as shallow and culturally empty.
Forum Romanum: Ammianus Marcellinus - Latin text of Res Gestae a Fine Corneli Taciti, as well as a list of related sites.
Histos: Ammianus Marcellinus on the Geography of the Pontus Euxinus - Paper by Jan Willem Drijvers of the University of Groningen considering Marcellinus' digression on the Black Sea in the Res Gestae as a literary rather than reference piece.
The Battle of Adrianople - Excerpt from the Yonge 1862 translation (History, XXXI.12-14), on the Roman defeat.